1. Field of the Invention
The present disclosure generally relates to image forming and, more particularly, to an image forming apparatus including a function for measuring the color of a measurement image.
2. Description of the Related Art
Factors of quality of images (hereinafter, referred to as image quality) of an image forming apparatus generally include graininess, in-plane uniformity, character quality, and color reproducibility (including color stability). Nowadays, multicolor image forming apparatuses are prevalent, and in the usage environment, color reproducibility is regarded as one of the most important factors of the image quality.
A person has memories of expected colors (especially, human skin, blue sky, metal, etc.) derived from experiences and may feel uncomfortable if the colors are beyond an allowable range thereof. Such colors are called memory colors, and the reproducibility of the colors becomes an important factor in outputting photographic images, and the like.
In addition to the photographic images, also in document images, the degree of demand for color reproducibility (including stability) to the image forming apparatuses is growing among office users who feel uncomfortable with differences from colors on monitors, and users of graphic arts who pursue color reproducibility in computer graphics (CG) images.
In order to fulfill such users' demands for color reproducibility, an image forming apparatus for reading a measurement image (patch image) formed on a sheet with a measurement unit (color sensor) provided on a conveyance path of the sheet is discussed (for example, see Japanese Patent Application Laid-Open No. 2004-086013). In the image forming apparatus, based on a read result of a patch image by the color sensor, feedback on process conditions such as an amount of exposure and developing bias is provided, thereby enabling reproducibility at a certain density, a certain gradation, and a certain tint.
In the color sensor described in Japanese Patent Application Laid-Open No. 2004-086013, however, the color detection accuracy decreases due to factors such as output variations of a light source caused by change in ambient temperature. To solve the problem, a white reference plate may be arranged at a position opposed to the color sensor, and calibration for measuring the white reference plate with the color sensor and correcting the detected value of the color sensor may be performed.
Specifically, a spectral reflectivity R(λ) of the patch image can be calculated by the following (equation 1).R(λ)=P(λ)/W(λ)  (equation 1).Where reflected light from the white reference plate is W(λ), and reflected light from a patch image is P(λ).
In the calculation of the spectral reflectivity of the patch image using the white reference plate, an error in the measurement value may occur due to color change of the white reference plate caused by the emitted light. If materials contained in the white reference plate include a material that changes color by oxidation by light, color change occurs due to light emission to the white reference plate.
During the calibration operation, light is emitted to the white reference plate, and as the number of times of the calibration increases, the color change gradually proceeds due to the emitted light, thereby increasing errors in measurement values.